An important factor in the safety of flight is the use of airborne weather radars for the detection and avoidance of regions of heavy precipitation, since: (1) they can be associated with turbulence and/or airflows that can be dangerous to the aircraft; and (2) the precipitation itself can be of sufficient magnitude to cause failures, such as engine flameout. Currently, airborne weather radars do not consistently measure precipitation intensity with a high degree of accuracy. Specifically, in a large population of aircraft, it is believed that a significant number of the airborne weather radars will overestimate or underestimate precipitation by as much as ten to one. This means that, in such cases, light precipitation will sometimes be displayed as dangerously heavy when it is not, and heavy precipitation will sometimes be displayed as light and not potentially dangerous when in fact it is dangerous. Such a calibration error is recognized as having contributed to some recent airlines crashes wherein very heavy precipitation apparently was not properly detected and displayed by the airborne weather radar.
This incorrect estimation of precipitation will occur because of the inadequacies of current practices for periodically calibrating airborne weather radars for precipitation intensity measurement purposes. In operation, a weather radar estimates precipitation by first measuring the "radar cross-section" of the precipitation and then converting this cross-section measurement to precipitation by an equation based on an assumed droplet size distribution of the rainfall, and a knowledge of the radar wavelength, pulse length and beam width. The equation itself is not considered to be a substantial source of error in determining and displaying precipitation according to this disclosure.
There are two basic techniques for calibrating a radar system for the purpose of measuring radar cross-section whether the radar is a weather radar or a general purpose radar designed to measure radar cross-section for purposes of determining the radar cross-section of a military or civil aircraft or other target.
The first basic technique, and the one used in current airborne weather radar practice, is to calculate or measure, all the individual radar parameters such as antenna gain, power output, receiver sensitivity, waveguide losses, etc., and to insert such parameters in an equation that relates a measured echo at a measured distance to the equivalent radar cross-section of the target generating the echo at that distance. The difficulty with this practice is that an accurate measurement of radar cross-sections requires, at the time of the measurement, an accurate knowledge of all the above parameters. In practice, of course, this is not possible because earlier-made measurements of power output, receiver sensitivity, etc. are assumed to be stable since the last measurement, which may no longer be true since these parameters change with time. For instance, waveguides can become corroded and produce losses that can significantly affect the cross-section measurement, and there are many other cumulative losses which are not readily detected by periodic conventional calibration procedures. By this technique it is very difficult, using routine calibration procedures, to obtain any absolute radar calibration to within .+-.3 db, and this calibration can easily vary by up to .+-.10 db in a large population of aircraft, even with frequent calibration procedures, since such procedures do not cover all possible sources of error such as waveguide losses. A 10 db error in calibration can generate a 10:1 error in precipitation intensity estimation.
The second basic technique is often used for calibrating ground based radars on an absolute basis to provide accurate measurement of radar cross-section. It usually involves a ground based radar observing a point target of known radar cross-section at a known distance. The radar indication is then calibrated directly in terms of radar cross-section. In this procedure, it is not necessary to individually calculate or measure parameters such as power output, receiver sensitivity, waveguide losses, etc., since such parameters are not needed for the calibration procedure. In order to use this initial calibration to further calibrate the radar for targets of different cross-sections appearing at different ranges it is only necessary to know the relationship of receiver signal output to echo intensity input and to apply a correction for range. This absolute calibration procedure is typically implemented for ground based radars by the use of a metallic sphere suspended from a balloon. "Calibration of a Weather Radar Using a Standard Target" Atlas & Massag. Bulletin of the American Meteorological Society, Vol. 41, No. 7 July 1960. A metallic sphere has a radar cross-section that is independent of its viewing angle and is essentially equal to its projected area. It is usually suspended from a balloon so that it will provide a readily identifiable echo isolated from ground clutter targets. The reflector, can however, in certain applications be mounted on the ground and coded, or identified, by some means to distinguish it from ground clutter targets. Such coding or identifying techniques can be by mechanical modulation of the reflector or by electrical modulation as in my Frequency Shift Reflector U.S. Pat. No. 3,108,275. While the above procedure for direct calibration is conceptually quite simple, one problem that presents itself during in-flight calibration is that, as a practical matter the "calibration" target must be located on the ground in a known region. It is thus in the midst of "ground clutter" targets and hence tends to be obscured by such targets except under unique conditions. Moreover such targets add to or subtract from its known radar cross-section, thus invalidating the intensity calibration. A potential way around this problem is to make the calibration target sufficiently large in radar cross-section so that it greatly exceeds normal ground clutter targets. Statistically, a large specular target of 10,000 square meters (40 dbsm) will be experienced once per square mile of urban area at low viewing angles. One must therefore expect that an aircraft making a 3 degree approach to an airport will probably see one such large target at the airport. "Specular" clutter is clutter typically associated with man-made structures, in contrast to distributed clutter which is generally associated with rocks, trees, and terrain irregularities. Distributed clutter is two to three orders of magnitude below the 10,000 square meter clutter discussed herein, and hence is not a problem. The concept of providing large radar targets on an airfield that will stand out above ground clutter is, of course, extensively investigated in the prior art.
One solution is to provide ground based reflectors tilted at useful angles with respect to a guidance path as shown in my U.S. Pat. No. 3,243,816 in which I teach the art of using ground based reflectors in order to provide vertical guidance to landing aircraft cooperatively with an airborne radar, such as a weather radar. In that patent, I did not recognize the difficulty caused by the multipath reflections off the ground perturbing the reflective properties of the reflectors and causing rapidly alternating signal enhancements and cancellations which are of such character as to make reflectors periodically disappear completely from the radar display. Such reflective perturbations would of course destroy the value of a reflector for calibration purposes. Similarly, many efforts have been made to use reflectors spaced along a runway to outline the runway merely for purposes of horizontal guidance. Programs, such as the Bendix Microvision program, and other current tests by the FAA for purposes of helicopter navigation, have confirmed that the reflective patterns so obtained are unstable in that the reflectors appear and disappear as the aircraft approaches touchdown. This phenomenon is undoubtedly due to multipath effects as noted herein.
Subsequent to the issuance of my U.S. Pat. No. 3,243,816 (supra) U.S. Pat. Nos. 3,716,855 and 3,729,737 to Asam have taught the use of multiple reflectors along the runway for range tracking and for vertical guidance, without recognizing the need for protecting the reflective properties of such reflectors from the effects of multipath.
U.S. Pat. No. 4,104,634 to Gillard et al does seek to solve the multipath problem experienced by dihedral corner reflectors used near the ground, by tilting the reflectors normal to the ground so that no spurious multipath problem is possible since all useful signals must be ground reflected. A very significant difficulty with this technique is that the ground surface must be made flat and of consistent and significant reflecting projection at radar frequencies over very large areas. This tends to be impractical in changing meteorological conditions i.e. snow, ice, rain, etc. Current low frequency (300 MHz) ILS vertical guidance beams experience this difficulty when their design is such as to use a reflection off the ground for beam formation purposes. More modern low and high frequency ILS vertical guidance systems use highly directional antennas seeking to eliminate such multiple reflections as a source of guidance error.